Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

Classmates at the Irish immigrant's Massachusetts high school called Prince a "whore" and an "Irish slut," students said. They defaced her school photo with obscene drawings, sent her threatening text messages and whispered — or shouted — insults in school hallways. On Jan. 14, witnesses say, she was taunted by a group of classmates in the library and hit with a can of Red Bull thrown from a moving car. That afternoon, Prince went home and hanged herself with a scarf.

Nine students have now been charged with harassment and other bullying-related crimes, spurring national debate about the role of the justice system and the culpability of the school administration. But Prince's case raises another, more elemental question: Why are kids so cruel?

Beginnings of bullying researchResearch into bullying didn't start until the 1970s, when psychologist Dan Olweus began to study the phenomenon in Norwegian schoolchildren. In fact, much of the study was triggered by the suicides of several young victims of bullying, said René Veenstra, a sociologist at the University of Groningen in the Netherlands.

Since then, decades of research have shown that the power differential between bullies and victims is a crucial component of the interaction.

"Bullies go for admiration, for status, for dominance," Veenstra said. Unlike friendly teasing, he said, bullying is long-term, unwanted and doesn't occur between social equals.

Despite their aggressive behavior, bullies also want affection, Veenstra said. His work has shown that bullies care about the approval of their own in-group, so they strategically pick victims they know few other classmates will defend.

Other researchers have found evidence that kids who are already socially awkward are more vulnerable to bullies. But there's no one thing that makes a child a target.

"There's actually no good reason," said Young Shin Kim, a professor at the Child Study Center at the Yale School of Medicine. "One day, they just don't like a kid because that kid will wear pink, and the next day they might not like other kids because they're wearing blue, or they're tall, or they're small, or they wear glasses. ... It's just not really, systemically, that there's some kind of reason or motivation. It's more like a cultural thing."

A culture of bystanders
For some kids, bullying behavior is just the tip of the iceberg, Kim said. These children have other problems with aggression and control and may be abuse victims themselves. But there are also many otherwise well-adjusted children who just "think it's a cool thing to do," she said.

Indeed, 85 percent of bullying cases happen for the benefit of an audience, Veenstra said. Bullies want their behavior to be noticed. That means the reactions of bystanders is another essential piece of the bullying puzzle.

"There are often defenders to the victims, but there are certainly more bystanders," Veenstra said. Other children have a difficult time intervening without the support of teachers and authority figures, who are sometimes too quick to dismiss bullying. And adults don't always set good examples. Take driving: Grown-ups often tailgate slow drivers in an effort to intimidate them, Kim said. That's a page right out of the bully handbook.

When it comes to bullying, Phoebe Prince's case was almost textbook. She was a new girl, different from her classmates, who dared to date a popular upperclassman, which allegedly drew the wrath of other popular kids who wanted to put her in her place. Bullying is often used to maintain the social pecking order, Veenstra said.

And while suicides by victims are rare, bullying does increase suicide risk. It can also cause poor school performance, depression and low self-esteem that persists for years.

Bullies, too, fall victim to their own behavior. They have higher risks of delinquency, substance abuse and psychological problems. One study of Korean schoolchildren found that all female students involved in bullying (whether as bully, victim or both) had higher rates of suicidal thoughts and behavior.

"Bullying experience is not something that you overcome without consequences," Kim said.

Bullying is also not inevitable. Anti-bullying programs work, researchers say. The Scandinavian countries, which implemented widespread anti-bullying curricula in the 1970s and '80s, now have some of the lowest bullying rates worldwide.

The key is that anti-bullying messages must be consistent and widespread, says Rosalind Wiseman, author of "Queen Bees and Wannabes: Helping Your Daughter Survive Cliques, Gossip, Boyfriends, and the New Realities of Girl World" (Three Rivers Press, 2003) and creator of the anti-bullying curriculum "Owning Up."

"Please don't waste anybody's time by doing a 45-minute bullying assembly, and then putting on some piece of paper that you have a zero-tolerance policy for bullying," Wiseman said. For the message to take, she said, teachers must be trained to respond to bullying on a daily basis, and the culture of the school must reinforce that bullying is not acceptable.

In the end, Kim said, one of the worst mistakes adults can make is to shrug off blame on the younger generation.

"We grown-ups have to be much more active, proactive and responsible and do something about it," she said. "It's not kids' problem. It's our problem."